The fabrication of semiconductor devices on semiconductor wafers requires that the semiconductor wafers be processed in a variety of different manners. Photolithography, chemical mechanical polishing (CMP), metal deposition, and so on, are all processes performed on semiconductor wafers in precise degrees to fabricate semiconductor devices. Usually, each of these different processes is performed using a different piece of semiconductor fabrication equipment. Therefore, a transport system is used to transport the semiconductor wafers among the different pieces of semiconductor fabrication equipment to achieve semiconductor device fabrication. Usually the semiconductor wafers are transported in pods, such as front-opening unified pods (FOUP's).
FIG. 1 shows a top view of a traditional pod transport system 100. The system 100 particularly includes a conveyor belt 104 on which the pods may be transported among various stations 102a, 102b, . . . , 102n. Each of these stations 102a, 102b, . . . , 102n may be a separate piece of semiconductor fabrication equipment, a storage place to store pods and their semiconductor wafers, and so on. Overhead transfer (OHT) units 106 are able to move the pods vertically. The OHT units 106 are also referred to as OHT grippers, in that they grip the pods.
One particular area 108 of the system 100 of FIG. 1 is shown in more detail in a side view in FIG. 2. One of the OHT units 106 has a pod 202a that it is vertically lowering onto one of the load ports 204. One of the robot arms 208 has lowered on its vertical axis 206 to pick up the pod 202b that has already been lowered onto one of the load ports 204. The other of the robot arms 208 has risen on its vertical axis 206 to deposit the pod 202c onto one of the conveyor belts 104. In this way, pods 202 are moved from the OHT units 106 to the load ports 204, and ultimately to the conveyor belts 104. Furthermore, the OHT units 106 can vertically raise the pods 202 from the load ports 204.
A difficulty with the system 100 is the manner by which the OHT units 106 engage the pods 202 from the load ports 204 for vertically raising them. In particular, the OHT units 106 typically have latches that rotatably engage into corresponding latch holes of the pods 202. However, if the latches of the OHT units 106 do not smoothly slide into the latch holes of the pods 202, the latches may jam in the latch holes of the pods 202. Removing the pods 202 from the OHT units 106 can in this case be accomplished only manually, by forcing the pods 202 from the OHT units 106. Besides the downtime that results from this problem, it can also damage the pods 202, and particularly their latch holes. The pods 202 may then have to be repaired or replaced.
Therefore, there is a need to overcome these disadvantages. There is a need to ensure that the latches of OHT units are smoothly slid into the corresponding latch holes of pods. There is also a need to ensure that damage does not result to the pods when the latches of the OHT units do not smoothly slide into the corresponding latch holes of pods. For these and other reasons, therefore, there is a need for the present invention.